1. Field of the Invention
This invention relates generally to equipment for cementing liners in well bores and specifically to a well bore liner cementing apparatus having multiple liner wiper plugs for wiping the interior surfaces of the operating string and liner during cementing operations.
2. Description of the Prior Art
A liner is a section of casing or tubing which is suspended in a well without normally extending to the surface. Cemented liners are used for many purposes including well control and reducing the initial cost of casing. Liners may be installed entirely within outer casing strings or partially within the casing and partially within an open hole.
Conventionally, a liner is set and cemented by first lowering the liner and a setting tool connected to an operating string into the well bore. The liner is hung, usually on slips, and the setting tool is usually, but not always released from the liner. Cement is then pumped through the operating string, into the liner, and displaced from the liner, usually through a foot valve, into the annular space between the liner and the surrounding casing or well bore.
In most cases, a pump down plug is introduced into the liner string immediately behind the cement in order to separate the cement from the displacing fluid and to wipe the cement from the operating string and liner surface as the cement is pushed out of the liner into the surrounding annular space. Typically, the pump down plug which is to wipe the operating string and liner is pumped behind the cement until it engages a liner wiper plug and then the liner wiper plug and pump down plug are forced downwardly together in the liner string so as to displace the cement therefrom and to wipe the liner walls.
U S. Pat. No. 3,910,349 to Joe R. Brown et al, entitled "APPARATUS AND METHOD FOR CEMENTING WELL LINERS", issued Oct. 7, 1975, shows a liner cementing apparatus which includes a setting tool having a tubular mandrel connected in a pipe string for extension through the liner. A liner wiper plug is releasably disposed within the liner near one end of the mandrel. After the liner is hung in position in the well bore, the setting tool is unlatched and moved axially a few feet to indicate to the operator at the surface that disengagement of the setting tool has occurred. A pump down plug engages the liner wiper plug to wipe the interior of the operating string and liner behind the cement column.
It is an advantage in well cementing operations to provide a cementing apparatus which utilizes multiple plugs to completely isolate the column of cement being pumped through the operating string and liner. U.S. Pat. No. 3,364,996, to C. C. Brown, entitled "APPARATUS FOR CEMENTING WELL LINERS", issued Jan. 23, 1968, shows a cementing apparatus utilizing a total of four plugs to isolate the column of cement being pumped from the well bore fluids both in front of and behind the column of cement. The present invention is an improvement to the device shown in U.S. Pat. No. 3,364,996, in that the top liner wiper plug is positively locked to the plug carrying body of the setting tool until the bottom wiper plug has been released. Both the top and bottom liner wiper plugs are positively locked to each other and to the setting tool until the pump down plugs land in their respective seats. Because of the positive lock features of the design, premature shear of either the top or bottom liner wiper plugs is prevented.
In co-pending U.S. patent application Ser. No. 7/147,699, filed Jan. 25, 1988, entitled "APPARATUS AND METHOD FOR CEMENTING A LINER IN A WELL BORE", there is shown and disclosed and apparatus and method for cementing a liner in a well bore utilizing a unique cement wiping plug system. While such system is unique for the reasons set forth therein, it has recently been discovered that said device may not be completely hydraulically balanced with respect to all of its shear releasing mechanisms in that for example with respect to the device shown in FIG. 1B of said application, the retainer ring 151 is held in place by a shear pin mechanism 155 which could be exposed to premature shearing as the bottom plug assembly travels downwardly within the liner subsequent to release from the upper plug, as described in said patent application by means of the encountering of the shoulder 147 of the sleeve 125 and a direct downward force being applied at the upper end 149 of the retainer ring 151 to thereby cause a premature load on the pin 155 and cause same to prematurely shear.
The present invention overcomes this possible problem by assuring that the sleeve mechanism never applies a direct load to the retaining member for the collet mechanism.